Semiconductor manufacturers are continuously working to improve the performance and reliability of power devices such as double diffused metal oxide semiconductor (DMOS) and insulated gate bipolar transistor (IGBT) devices. One of the areas of concern is adhesion failure of the bond pads for the power device die. The bond pads are located on a top surface of the die and include a metal layer that is formed over a thick oxide or field oxide region of the die. The bond pads are wire bonded to inner leads of a package lead frame to form electrical contacts between the bond pads and the package lead frame.
One approach that has been used to address the issue of adhesion failure is to form a layer of polysilicon that is between the metal layer and the field oxide. The polysilicon layer provides a chemically compatible interface between the metal layer and the field oxide thereby improving adhesion. The polysilicon layer absorbs external thermo-mechanical stress and improves durability against stress in a vertical direction relative to a surface of the metal layer. The field oxide however can have defects caused during formation that are areas of localized thinning that are randomly distributed throughout the field oxide. Power devices typically operate at high voltage levels, such as 250 V to much higher than 1000 V. Having the polysilicon layer in direct contact with these defect areas when a high voltage level is applied to the bond pad metal layer during operation can result in a high current or short circuit condition that will destroy the power device. The adhesion and stress absorption benefits of using polysilicon over the relatively large surface areas required for bond pads can be offset by the random distribution of the localized thinning defects and the increased probability of a high current condition occurring that can destroy the power device
Another approach is to form the metal layer directly over the field oxide. Metals such as aluminum have good adhesion to silicon and to silicon oxides such as SiO2. To avoid diffusion of silicon into the aluminum, silicon must be added to the aluminum layer. Because the addition of silicon to the aluminum metal layer can result in problems such as silicon grain growth, another approach is to use a diffusion barrier between the aluminum metal layer and the SiO2 field oxide. Diffusion barrier layers such as a titanium (Ti), titanium nitride (TiN) or titanium tungsten (TiW) are effective at preventing the diffusion of silicon into the aluminum metal layer. Barrier layers formed from Ti/TiN or TiW however tend to have poor adhesion to SiO2. Due to the relatively large area of the SiO2 field oxide needed for the bond pad metal and barrier layers, and the different thermal expansion properties of the material layers used to form the bond pad, lateral stress in a direction parallel to the surface of the band pad can result in peeling or separation of the bond pad material layers from the SiO2 field oxide.